Thianthrene as an activating group for the synthesis of poly(aryl ether thianthrene)s by nucleophilic aromatic substitution

A method for the preparation of poly(aryl ether thianthrene)s has been developed in which the aryl ether linkage is generated in the polymer‐forming reaction. The thianthrene heterocycle is sufficiently electron‐withdrawing to allow fluoro displacement with phenoxides by nucleophilic aromatic substitution. The monomer for this reaction, 2,7‐difluorothianthrene, can be synthesized in a moderate yield by a simple reaction sequence. Semiempirical calculations at the PM3 level suggest that 2,7‐difluorothianthrene is sufficiently activated, whereas NMR spectroscopy (¹H and ¹³C) indicates that the monomer is only slightly activated or (¹⁹F) not sufficiently activated for nucleophilic aromatic substitution. Model reactions with p‐cresol have demonstrated that the fluorine atoms on 2,7‐difluorothianthrene are readily displaced by phenoxides in high yields, and the process has been deemed suitable for polymer‐forming reactions. High‐molecular‐weight polymers have been produced from bisphenol A, bisphenol AF, and 4,4′‐biphenol. The polymers have been characterized with gel permeation chromatography, NMR spectroscopy, differential scanning calorimetry, thermogravimetric analysis, and matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry. The glass‐transition temperatures for the polymers of different compositions and molecular weights range from 138 to 181 °C, and all the polymers have shown high thermooxidative stability, with 5% weight loss values in an air environment approaching 500 °C. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 6353–6363, 2004     

圆柱面上微小后向台阶层流绕流场的数值研究

1前言钩体绕流是燃烧、传热领域基本的流动现象，具有流动分离、流态转被、旋涡脱落、涡致振动等复杂特性。降低钩体绕流阻力一直是人们关心的问题。自从Prandtl圆球绊线实验以来，Stuper，Roshko，Zukausks，Achenbach，Klebanoff等［‘一列从不同角度研究了钝体绕流阻力的构成因素及降低钝体绕流阻力的若干方法。一致的观点是：因绕流钝体必然伴随着流动分离，流动阻力可分为与粘性有关的摩擦阻力及与流动分离有关的压差阻力两部分，其中，压差阻力约占总阻力的90N95％，其大小与流动分离点的位置密切相关。若流动分离时边界层内为层流…     

关于图P_n~k的均匀染色

对简单图G=〈V,E〉及自然数k,令V(Gk) =V(G) ,E(Gk) =E(G)∪{uv|d(u,v) =k},其中d(u,v)表示G中u,v的距离,称图Gk为G的k方图.本文讨论了路的k方图Pkn的均匀点染色、均匀边染色和均匀邻强边染色,利用图的色数的基本性质和构造染色函数的方法,得到相应的色数χev(Pkn) ,χ′ee(Pkn) ,χ′eas(Pkn) .并证明猜想“若图G有m -EASC,则一定有m +1 -EASC”对Pkn是正确的.     

Finite element analysis of edge lamination during hot rolling process of aluminium alloy

Summary The rigid-plastic finite element method is applied to the analysis of the three-dimensional deformation behaviour, in particular the forming of edge lamination during the multi-pass hot roughing process of aluminium. the calculation is carried out for various rolling conditions and reduction schedules, and their influence on the edge lamination is examined. The results suggest that the crop loss due to the edge lamination can be reduced by increasing the roll diameter, the reduction per pass and the initial width of the slab.     

一种改进的SAR图像边缘检测方法

针对Ration边缘提取算法对弱边缘检测效果差，边缘定位精度低的缺点，提出了一种改进算法用于检测微弱边缘，并给出具有FCAR特性的阀值选取办法，实际应用中，将两种方法结合为一种综合处理方法，对真实SAR图像的处理结果表明，该综合处理方法具有较高检测正确率及较好的边缘定位精度。     

星载红外分光计的扫描过渡控制

采用电子控制方法实现对星载红外分光计扫描镜步进过渡过程的控制.解决了扫描测量过程中光学通道在其测量周期内视场的变化,保证了各测量通道的空间同一性.     

Spectral-fractional step Runge-Kutta discretizations for initial boundary value problems with time dependent boundary conditions

In this paper we develop a technique for avoiding the order reduction caused by nonconstant boundary conditions in the methods called splitting, alternating direction or, more generally, fractional step methods. Such methods can be viewed as the combination of a semidiscrete in time procedure with a special type of additive Runge-Kutta method, which is called the fractional step Runge-Kutta method, and a standard space discretization which can be of type finite differences, finite elements or spectral methods among others. Spectral methods have been chosen here to complete the analysis of convergence of a totally discrete scheme of this type of improved fractionary steps. The numerical experiences performed also show the increase of accuracy that this technique provides.

     

Partitioning series-parallel multigraphs into υ*-excluding edge covers

We prove that, for any given vertex υ* in a series-parallel graph G, its edge set can be partitioned into k = min{κ'(G) + 1,δ(G)} subsets such that each subset covers all the vertices of G possibly except for υ*, where δ(G) is the minimum degree of G and κ'(G) is the edge-connectivity of G. In addition, we show that the results in this paper are best possible and a polynomial time algorithm can be obtained for actually finding such a partition by our proof.     

激光二极管正向电特性的精确检测

采用正向交流特性结合I-V特性的方法,检测了激光二极管的串联电阻、理想因子、结电压和结电容与外加电压或电流的关系.首次发现,激光二极管的结电压、串联电阻、理想因子和结电容在阈值附近同时出现了明显的阶跃,之后结电压呈现饱和.此外还观察到,在较低的测试频率和较大的正向电压下,激光二极管的结电容具有负值.     

Facet-edge fluctuations with periphery diffusion kinetics

We investigate the novel scaling of the steps bounding a facet surrounded by a rough region. The hindered, asymmetric fluctuations can be associated with the emergence of a dominant non-linear term in the Hamiltonian governing the step fluctuations. We explore the crossover from unhindered to hindered fluctuations, calculating the growth exponent, β, with Monte Carlo simulation within the TSK model. The hindered behavior is found in the simulations when the facet-edge step is separated by fewer than six atomic spacings from the second step. Actual fluctuations are larger than in this calculation, particularly at higher temperatures, making the hindered behavior easier to observe. In addition, we discuss the possibility that volume conservation effects in nanoscale structures may cause similar confinement in non-conserved fluctuations.